The present invention generally relates to a method for interconnecting a flat panel display and a device formed and more particularly, relates to a method for interconnecting a liquid crystal display panel to a printed circuit board wherein the display panel has a non-transparent substrate and a device formed by the method.
In recent years, liquid crystal display (LCD) panels have been used widely in place of cathode ray tubes (CRT) in electronic display applications. The LCD panel is first assembled together by filling a liquid crystal material in-between a LCD substrate and a transparent glass cover plate. The LCD substrate consists of a multiplicity of switching, or electronic turn-on and turn-off devices for operating the multiplicity of pixels formed on the LCD panel.
After the assembly of a LCD panel is completed, the panel must be connected to an outside circuit for receiving electronic signals such that images may be produced in the panel. The electronic connections to the LCD panel can be provided by different techniques. A conventional technique for providing electronic signals to a LCD panel is by using a flexible printed circuit board (FPCB) which contains various electronic components welded thereto for providing signals to the LCD. The flexible printed circuit board is frequently manufactured of a conductive copper layer sandwiched between two flexible polyimide cover layers. The flexibility of FPCB is advantageous in the installation of a LCD panel. FIG. 1A is a graph illustrating a conventional design of LCD panel 10 connected by a FPCB 12 to a printed circuit board (PCB) 14 which has a surface mount technology (SMT) type IC chip 16 mounted on top.
In another conventional technique for bonding a LCD to a PCB, as shown in FIG. 1B, a tape automated bonding (TAB) technique is used. In the TAB bonding technique, a TAB section 20 is used to connect LCD 10 and PCB 14. The TAB section 20 consists of a TAB tape 22 which has an IC chip 16 connected to it through bonding sites 24. The TAB bonding technique provides the benefit of a compact package so that circuit density can be improved resulting in a lead pitch as low as 60 xcexcm. The TAB, also known as TCP (tape carrier package) utilizes finely patterned thin metal, i.e., copper foil plated with Au or Sn, in place of wires and connects the metal tips metallurgically to corresponding gold plated bumps that are formed on the aluminum pads on the chip. TAB is preferred in smaller-pitch interconnects for high I/O ULSI devices because it enables smaller pitch and longer span bondings than those available by wire bonding. However, the TAB bonding technique is normally conducted at a higher fabrication cost.
In still another technique for bonding a LCD to a PCB, as shown in FIG. 1C, a chip on glass (COG) technique is used. In the COG technique, an IC chip 16 is mounted directly on a LCD 10 by utilizing solder bumps 24 and an anisotropic conductive film (ACF) 26. Detailed cross-sectional views of an ACF 26 is shown in FIGS. 2A and 2B. As shown in FIG. 2A, a TAB tape 22 which has conductive pads 28 formed on top is positioned over an ACF tape 30 which contains electrically conductive particles 32 embedded an insulative compound 34. Positioned under the ACF 30 is a LCD substrate 10 which has conductive elements 36 formed on top. After the TAB tape 22, the ACF 30 and the LCD substrate 10 are pressed together under heat, as shown in FIG. 2B, the conductive particles 32 provides electrical communication between the conductive pads 28 and the conductive elements 36 and therefore allowing the TAB tape 22 to electrically communicate with the LCD substrate 10. It should be noted that, electrical communication between the conductive pads 28 and the conductive elements 26 is only established where the conductive particles 32 are compressed, i.e., only established anisotropically and selectively. The conductive elements 36 on the LCD substrate 10 is normally formed of indium-tin-oxide (ITO) thin films.
As shown in FIG. 1C, the COG technique may further connect the LCD substrate 10 to a printed circuit board (not shown) by a flexible printed circuit board (not shown). The COG technique therefore relies on bonding with solder bumps 24 formed on an IC chip and the ACF for electrical communications.
Each of the three conventional techniques for forming a TFT-LCD assembly has its benefits and disadvantages. For instance, in the first technique of using SMT/FPCB, the circuit density can be increased to achieve a compact package at the expense of using difficult TAB technology and high material costs. In the TAB and COG method, a rework of the assembly such as the removal of a defective IC from a LCD substrate is extremely difficult, if not impossible. For instance, the only possible means for removing an IC chip that is bonded to a LCD substrate is by using a shear force for pushing an IC chip and breaking its bond with the LCD substrate. This is a difficult process and frequently results in the destruction of the entire assembly.
In the present fabrication process for TFT-LCD assemblies, the SMT/FPCB method is frequently used in fabricating lower priced assemblies such as those utilizing small LCD panels. In large LCD panel applications, i.e., such as those used in notebook computers, the TAB bonding method is normally used. The COG method, due to its difficulty in reworking and repair, is also limited to small LCD panel display applications. The TAB process and the COG process are therefore the two major assembling methods used for TFT-LCD assemblies. To sum up, the TAB method can be easily reworked and repaired by removing an IC chip from the TAB tape and furthermore, it is compact in size which allows the achievement of high density packages of up to 60 xcexcm pitch. However, the TAB process requires complicated fabrication steps which include IC bonding, tape fabrication, inner lead bonding, encapsulation, outer lead bonding and the ACF process. Another drawback for the TAB process is, during the ACF processing, there is a thermal expansion problem which may lead to misalignment between the leads. The TAB tape may further absorb moisture and contribute to its dimensional instability. Elaborate equipment may also be required for the TAB process and therefore increasing its fabrication costs.
In the COG process, while the fabrication steps required are simpler, i.e. only IC bumping and ACF processes are required, and furthermore, there is no thermal expansion problem and smaller pitch such as 50 xcexcm can be achieved, the fabrication of an LCD package of compact size is nevertheless difficult. This difficulty is illustrated in FIGS. 3A and 3B. In FIG. 3A, a cross-sectional view of an LCD package 40 which consists of an LCD substrate shown as a lower glass panel 10, an upper glass panel 42, an IC chip (or a driver chip) 16, an anisotropic conductive film (ACF) layer 26, and a printed circuit board (or a flexible printed circuit board) 14 is shown. The driver chip 16 is equipped with a multiplicity of solder bumps 44 formed on an active surface of the chip 16 for making electrical connection with a second multiplicity of conductive leads 46 (such as ITO leads) formed on the top surface 48 of the lower glass panel 10. The electrical communication is established by using the anisotropic conductive film layer 26 which is loaded with electrically conductive particles 50. In order to communicate with the outside circuits, such as circuits on the PCB 14, electrical communication through a third multiplicity of conductive pads 52 is provided between the PCB 14 and the lower glass panel 10. A plane view of the LCD package 40 is further shown in FIG. 3B.
As shown in FIGS. 3A and 3B, the lower glass panel 10 of the LCD package 40 is normally provided with a larger area than the upper glass panel 42 and thus leaving an exposed edge area 60 necessary for mounting of the driver chip 16 and the PCB 14. The excess edge portion 60 therefore cannot be reduced which results in excessively large LCD packages that are not suitable for certain applications that require a compact LCD package. The technology of connecting driver chips and external PCB circuits to an LCD package therefore must be improved in order to produce compact LCD panels.
More recently, a flat panel display package has been developed wherein one of the glass panels is replaced by a non-transparent panel such as a silicon substrate. The display module formed is known as a liquid crystal on silicon, or LCoS in abbreviation. In a typical liquid crystal on silicon module, the interconnection between the liquid crystal and a printed circuit board is by wire bonding or by an anisotropic conductive film (ACF) placed on silicon. A typical liquid crystal on silicon module 70 is shown in FIG. 4. The liquid crystal on silicon module 70 is constructed by a glass substrate 54 and a silicon substrate 56 with a liquid crystal material 58 filled therein-between. Spacers 62 are utilized to keep a constant space between the two substrates. An adhesive bead 64 is used along a perimeter of the glass substrate 54 for sealing and retaining the liquid crystal material 58. When the liquid crystal display panel 66 is used to interconnect to a substrate such as a printed circuit board 68, the silicon substrate 56 is adhesively bonded to the printed circuit board 68. IC chips 72 are mounted on top of the printed circuit board 68 for providing driver circuits to the liquid crystal panel 66. The electrical interconnections are thus made by wire bonds 74 between the liquid crystal panel 66 and the printed circuit board 68. Bond pads 76 and metal traces 78 are further provided for accomplishing such interconnections.
In the liquid crystal on substrate module 70 shown in FIG. 4, while the wire bonding technology has matured and can be used reliably in producing liquid crystal display modules in an automated process, the throughput of the process is low due to the time consuming wire bonding process. Other processing and design difficulties such as the module cannot be aligned in an existing alignment equipment which is designed for aligning modules that have a transparent base plate, or the poor heat dissipation since a heat sink cannot be directly attached to the silicon substrate. These processing and design limitations greatly limit the application potential of the liquid crystal on silicon modules.
It is therefore an object of the present invention to provide a method for interconnecting a flat display panel that has a non-transparent substrate without the drawbacks or shortcomings of the conventional interconnecting method.
It is another object of the present invention to provide a method for interconnecting a flat display panel that has a non-transparent substrate without using the wire bonding technique.
It is a further object of the present invention to provide a method for interconnecting a flat display panel that has a non-transparent substrate by using a glass panel that is larger than the non-transparent panel in forming the liquid crystal panel.
It is another further object of the present invention to provide a method for interconnecting a flat display panel that has a non-transparent substrate by utilizing anisotropic conductive film in interconnecting the liquid crystal panel to a printed circuit board.
It is still another object of the present invention to provide a method for interconnecting a flat display panel that has a non-transparent substrate to a flexible printed circuit board by utilizing an anisotropic conductive adhesive in-between the circuit board and the liquid crystal panel.
It is yet another object of the present invention to provide a flat display module formed by a flat panel electrically connected to a printed circuit board without any wire bonds.
It is still another further object of the present invention to provide a flat panel display module formed by a flat panel electrically connected to a printed circuit board by utilizing an anisotropic conductive film or an anisotropic conductive adhesive in-between the flat panel display and the printed circuit board.
It is yet another further object of the present invention to provide a flat panel display module formed by a flat panel electrically connected to a printed circuit board that can be processed in a conventional bonder and an alignment equipment.
In accordance with the present invention, a method for interconnecting a flat panel display that has a non-transparent substrate to a printed circuit board and a flat panel display module formed by such method are disclosed.
In a preferred embodiment, a method for interconnecting a flat display panel that has a non-transparent substrate can be carried out by the operating steps of first providing a flat display panel that is formed of a glass panel, a silicon substrate and a liquid crystal material filled therein-between; the glass panel has a length larger than a length of the silicon substrate resulting in an overhang on at least one side of the glass panel in a longitudinal direction; providing a multiplicity of metal leads on an edge portion of the silicon substrate not covered by the liquid crystal material in electrical communication with a multiplicity of thin film transistors; providing a multiplicity of transparent conductive traces on the overhang of the glass panel; providing a printed circuit board that has a multiplicity of conductive pads formed on an edge portion of the board; positioning the flat display panel with the glass panel facing downwardly and the silicon substrate juxtaposed to the PCB; positioning an electrically conductive adhesive in-between the multiplicity of transparent conductive traces, the multiplicity of metal leads, and the multiplicity of conductive pads; aligning the flat display panel to the PCB optically from an underside of the panel through the glass panel; and compressing the PCB against the overhang of the glass panel such that the multiplicity of conductive pads on the PCB is electrically connected to the multiplicity of metal leads on the edge portion of the silicon substrate through the multiplicity of transparent conductive traces on the glass panel and the electrically conductive adhesive positioned therein-between.
The method for interconnecting a flat display panel that has a non-transparent substrate may further include the step of curing the electrically conductive adhesive by UV radiation from the side of the glass panel, or the step of selecting an electrically conductive adhesive from the group consisting of silver paste, electrically conductive elastomer, anisotropic conductive film, anisotropic conductive adhesive, metal bumps and solder balls. The method may further include the step of providing a multiplicity of transparent conductive traces that is formed of indium-tin-oxide, or the step of providing a printed circuit board that is a flexible printed circuit board, or the step of aligning the flat panel display to the PCB optically by a charge coupled device (CCD).
The present invention is further directed to a method for simultaneously forming a flat display panel and bonding to a printed circuit board which can be carried out by the operating steps of providing a silicon wafer that has a multiplicity of thin film transistors each formed on a multiplicity of dies; coating and buffing an alignment layer of a polymeric material on top of the silicon wafer achieving a preferred orientation of the material; mounting a multiplicity of spacers each having a preset thickness on top of the wafer; singulating the multiplicity of dies from the wafer and testing for reliability; positioning a frame seal along a periphery of one of the multiplicity of dies; forming a multiplicity of metal leads on an edge portion of the die electrically connecting to a multiplicity of thin film transistors; positioning a glass plate that has a larger area than the die resulting in an overhang on the die; forming a multiplicity of transparent conductive traces in the overhang on the die; providing a printed circuit board that has a multiplicity of conductive pads formed on an edge portion of the board; positioning the die and the glass plate with the frame seal therein-between in an upside-down position with the glass plate facing downwardly and the die juxtaposed to the PCB; positioning an electrically conductive material in-between the multiplicity of transparent conductive traces and the multiplicity of metal leads, the multiplicity of conductive pads; aligning the glass plate to the die and the PCB from an underside of the glass panel; compressing the PCB against the overhang of the glass plate such that the multiplicity of conductive pads on the PCB is electrically connected to the multiplicity of metal leads on the die through the multiplicity of transparent conductive traces on the glass plate and the electrically conductive material positioned therein-between; and filling a liquid crystal material between the glass plate and the die contained by the frame seal.
The method for simultaneously forming a flat display panel and bonding to a printed circuit board may further include the step of annealing the electrically conductive material by UV radiation applied from the side of the glass plate, or the step of providing the printed circuit board in a flexible printed circuit or the step of aligning the glass plate to the die and the PCB by a charge coupled device. The method may further include the step of compressing the PCB against the overhang of the glass plate between an upper platen and the glass plate, or the step of mounting a heat sink to the die for enhanced heat dissipation, or the step of providing the frame seal in a UV curable polymeric material. The method may further include the step of selecting the electrically conductive material from the group consisting of silver paste, electrically conductive elastomer, anisotropic conductive film, anisotropic conductive adhesive, metal bumps and solder balls.
The present invention is still further directed to a flat panel display (FPD) module formed by a flat panel electrically connected to a printed circuit board that includes a flat panel formed of a glass plate, a silicon substrate and a liquid crystal material filled therein-between, the glass plate has a length larger than a length of the silicon substrate resulting in an overhang on at least one side of the glass plate in a longitudinal direction; a multiplicity of metal leads on an edge portion of the silicon substrate not covered by the liquid crystal material in electrical communication with a multiplicity of thin film transistors; a multiplicity of transparent conductive traces on the overhang of the glass plate; and a printed circuit board that has a multiplicity of conductive pads formed on an edge portion of the board; the flat panel with the glass plate facing downwardly and the silicon substrate juxtaposed to the PCB is bonded to the PCB by an electrically conductive material placed in-between the multiplicity of transparent conductive traces, the multiplicity of metal leads, and the multiplicity of conductive pads; such that the multiplicity of conductive pads on the PCB is electrically connected to the multiplicity of metal leads on the edge portion of the silicon substrate through the multiplicity of transparent conductive traces on the glass plate and the electrically conductive material.
In the flat panel display module formed by a flat panel electrically connected to a printed circuit board, the multiplicity of transparent conductive traces may be formed of indium-tin-oxide (ITO). The electrically conductive material for bonding the flat panel to the PCB may be selected from the group consisting of silver paste, electrically conductive elastomer, anisotropic conductive film, anisotropic conductive adhesive, metal bumps and solder balls. The printed circuit board may be a flexible printed circuit. The electrically conductive material may be a UV curable material. The flat panel display module may further include a heat sink that is mounted to the silicon substrate for enhanced heat dissipation.